1. Field of the Invention
The present invention relates to corrosion detection. More specifically, the invention relates to a method for detecting and determining corrosion properties, such as type, location, size, and growth rate in metals and the like in a corrosive environment using a corrosion sensing probe. The invention is also capable of distinguishing between different types of corrosion such as uniform corrosion, and localized corrosion, such as pitting, crevice, and stress corrosion cracking.
2. Description of the Related Art
It has long been known that various forms of corrosion exist which result in the destruction of corrodible materials such as piping, tanks and other metal structures. Two major types of corrosion are uniform and localized corrosion. Uniform corrosion generally includes corrosion of large areas of a corrodible material at a roughly uniform rate. Localized corrosion, such as pitting or cracking, is generally smaller scale corrosion which is harder to detect. Localized corrosion occurs initially in a microscopically small area on a material surface, which eventually becomes larger and deeper, forming pits or cracks in the surface. Localized corrosion, particularly pitting, is hazardous because material is removed in a concentrated area that is not easily recognized. One of the most dangerous consequences of pitting corrosion is a leak in a containment vessel such as a tank or pipeline. The leak typically occurs at a pinhole in a wall of a containment vessel. The majority of the wall will have adequate thickness to contain the vessels contents. However, the resulting leak can be especially dangerous where the contained material is under pressure, at high temperature, or both.
Several methods have been. known in the art to detect corrosion of metals. However, these known methods suffer from various disadvantages such as corruption by environmental noise, a requirement of tuning for each application, or an inability to distinguish between different types of corrosion.
One known method involves the analysis and monitoring of electrochemical noise (ECN), or noise generated by the corrosion process. Various statistical analyses may be performed on these noise signals to distinguish corrosion type and corrosion rate in a corroded article. Examples of this ECN approach are described in U.S. Pat. No. 5,139,627, U.S. Pat. No. 5,425,867, and U.S. Pat. No. 6,015,484. There are, however, disadvantages of this ECN method. Electrochemical noise generated by localized corrosion is small and difficult to detect. Environmental noises from motors, circuit breakers, switches, and radio frequency generators in or near the corroded article may mask or obscure the corrosion noise signal, resulting in inaccurate ECN readings.
Another known method involves the use of a radioactive probe to detect pitting corrosion as described in U.S. Pat. No. 4,412,174. According to this method, a radioactive probe is placed into a corrosive stream. As pitting corrosion occurs, pieces of the radioactive probe break off and enter into the stream. The presence of radioactive material is detected by a downstream radiation detector, which allows for the analysis of the probe""s corrosion rate. A disadvantage of this approach is that it does not have the capability to differentiate between different types of localized corrosion, such as crevice and pitting.
Acoustic emissions can also be used to detect surface corrosion of an insulated pipe. This method is described in U.S. Pat. No. 5,526,689. This approach involves a transmission of acoustic emissions along the surface of an insulated pipe to locate corroded areas of the pipe. This approach, however, does not include the ability to distinguish between different types of corrosion.
U.S. Pat. No. 5,719,503 involves electromagnetic pulse propagation. According to this method, two electromagnetic sensors are mounted onto a corrodible article. Two pulses are sent from these sensors, and anomalies are examined at the intersection of the two pulses to locate areas of corrosion on the article itself. This approach also does not include the ability to distinguish between different types of corrosion.
It would be desirable to devise a method for corrosion detection which is accurate, reliable, and is able to distinguish among different types of corrosion such as stress corrosion cracking, uniform corrosion, and localized corrosion, such as pitting and cracking.
The present invention provides a solution to this problem. According to the invention, a corrosion sensor is provided which comprises a metal probe attached to a transducer element. The metal probe is inserted into a corrosive environment together with a corrodible article to be tested. The probe and the corrodible article are composed of a substantially identical metal material. A transducer element attached to the probe sends an ultrasonic or radio frequency signal through the probe, and receives any ultrasonic or radio frequency signals which have been reflected by corroded areas of the probe. These reflected signals are analyzed to determine the type, size, location, and growth rate of corrosion conditions in the probe, and thus the corrodible metal article.
The invention provides a method for detecting corrosion conditions of a corrodible metal article in a corrosive environment which comprises:
a) placing a corrosion sensor into the corrosive environment, which corrosion sensor comprises:
i) a metal probe comprised of a metal which is substantially identical to that of the corrodible metal article; and
ii) a transducer element attached to said probe, which transducer element is capable of projecting and receiving ultrasonic or radio frequency signals through the probe;
b) projecting ultrasonic or radio frequency signals from the transducer element through the probe; and
c) receiving reflected ultrasonic or radio frequency signals with the transducer element, which reflected ultrasonic or radio frequency signals are reflected by corroded areas of the probe, and generating an electrical response signal to the reflected ultrasonic or radio frequency signals, which indicates a corrosion condition of the probe.
The invention further provides a method for detecting corrosion conditions of a corrodible metal article in a corrosive environment which comprises:
a) placing a corrosion sensor into the corrosive environment, which corrosion sensor comprises:
i) a metal probe having an elliptical or circular cross section, which probe is comprised of a metal which is substantially identical to that of the corrodible metal article;
ii) a metal crevice ring:attached around the probe; and
iii) a transducer element attached to said probe, which transducer element is capable of projecting and receiving ultrasonic or radio frequency signals through the probe;
b) projecting ultrasonic or radio frequency signals from the transducer element through the probe;
c) receiving reflected ultrasonic or radio frequency signals with the transducer element, which reflected ultrasonic or radio frequency signals are reflected by corroded areas of the probe, and generating an electrical response signal to the reflected ultrasonic or radio frequency signals, which indicates a corrosion condition of the probe;
d) collecting a series of electrical response signals with a computer processor electrically connected to the transducer element; and
e) determining a corrosion condition of the probe and the corrodible article from the series of electrical response signals.
The invention still further provides a corrosion sensor for detecting corrosion conditions of a corrodible metal article in a corrosive environment, which corrosion sensor comprises:
a) a metal probe comprised of a metal which is substantially identical to that of the corrodible metal article to be tested; and
b) a transducer element attached to said probe, which transducer element is capable of projecting and receiving ultrasonic or radio frequency signals through the probe.